The present invention relates to recording/reproducing method and apparatus being able to record and reproduce information onto/from an optical disc, which forms a land and a groove on a recording surface thereof, and in particular, a focus-servo technique thereof for achieving recording/reproducing with high quality and stability, when conducting recording/reproducing on the disc rotating at a high velocity thereof, irrespective of a radius direction thereon, upon a presumption of a disc velocity being constant in rotation velocity thereof, such as, CAV (i.e., Constant Angular Velocity) and/or ZCAV (i.e., Zoned Constant Angular Velocity).
An optical disc of a disc-like optical information recording medium, such as, CD or DVD, etc., for example, is widely used as an information recording medium, because of characteristics or abilities thereof, i.e., non-contact, a large memory capacity, a low-cost, and enabling data access at high speed, for example, for recording/reproducing of digital audio data and/or digital video data, and further, as a data recording medium for a personal computer. In recent years, such an optical disc of rewritable type, as DVD-RAM (i.e., Digital Versatile Disc of Random Access Memory Type), for example, has been utilized widely.
With such the DVD-RAM disc, a recording medium of phase-changeable type is applied as a recording layer thereof. To increase the recording capacity thereof, there is applied so-called a single track format, i.e., a land and a groove appear, alternately, per one (1) rotation thereof, so that recording/reproducing can be made not only on the groove, but also on the land defined between the grooves.
However, since the optimal focus offset differs between the groove and the land, an operation is conducted by exchanging the focus offset at the position where exchanging is made between the groove and the land, in case when recording or reproducing by means of an optical pickup.
However, accompanying with development of high speed DVD-RAM in recent years, also the time period is shortened when a laser spot irradiated from the optical pickup passes through the exchanging portion between the groove and the land. For this reason, it is difficult to maintain a response speed of a focus actuator for the optical pickup, necessary for conducting or completing the exchanging operation of the focus offset at that switchover or exchange position, and in particular, in the operation of high-speed recording or high-speed reproducing higher than an eight-speed (i.e., 8X), there occurs a problem that the information recorded or reproduced is degraded or deteriorated in quality thereof.
Conventionally, as countermeasure of this, a technology is already known or disclosed in the following Patent Document 1, for example, as an improvement when exchanging between the focus offset at the position where exchanging is made between the groove and the land; i.e., adding a correction onto the optimal focus offset (Gopt) for the groove, or the optimal focus offset (Lopt) for the land, by a predetermined amount thereof, and thereby making the difference small, between those Gopt and Lopt.
Also, in the following Patent Document 2 is disclosed a technology of providing an equalizer means for boosting RF signals from the optical disc, and a demodulation means for demodulating data based on the signals from the equalizer means mentioned above, as a countermeasure for the deterioration of the reproduced signal, which accompanying exchange or switchover of the focus offset.
Patent Document 1: Japanese Patent Laying-Open No. 2002-319155; and
Patent Document 2: Japanese Patent No. 3,661,623.
As was mentioned, since no such presumption was made on the disc velocity (i.e., variable speed), with those conventional arts mentioned above, that the rotation velocity is always constant irrespective of the radius portion on the disc, therefore it is impossible to protect the information, which is recorded or reproduced under such variable speed of the disc, from being degraded or deteriorated in the quality thereof, fully. In addition thereto, since variation (or shortening) of rotation speed due to the variable speed of the disc is remarkable, as the disc rotation speed increases to be high, there is necessity of speed responding to the recording or the reproducing, however it is difficult to follow such variable speed of the disc with the conventional arts mentioned above.